Holiday calendar: A scientific slant on winter's start

A black-and-white photo from the EUMETSAT weather satellite shows Earth's terminator line at noon ET, just before the December solstice. The slanting line shows that the Southern Hemisphere receives more sunlight than the Northern Hemisphere in December, due to our planet's tilt.EUMETSAT

Earth spins like an tipped gyroscope as it makes its yearly circuit around the sun, which means some parts of the world can get more exposure to sunlight than others, depending on the time of year.

Seasonal changes are marked by the June and December solstices, when the differences in solar exposure are most extreme; and the March and September equinoxes, when Earth's axis is positioned such that north and south get equal exposure. Those yearly milestones were so important that bygone civilizations (including Stonehenge's builders as well as ancient peoples who lived in Peru, Brazil and Guatemala) erected astronomical observatories to keep track of them.

Meteosat 10 belongs to a class of satellites that take in an unchanging view of Earth's disk. That makes it easy to show how the balance of daytime and nighttime changes over the course of the year. You can get a similar picture by looking at satellite imagery from the GOES-13 weather satellite.